As is well known to those skilled in the art, it is important to maintain a bed of ion exchange resin covered by liquid at all times. Otherwise the resin may be damaged and the quality and quantity of the liquid passed through the bed are impaired.
The volume of some ion exchange resins varies in relation to the osmotic pressure of the solution in which they are immersed. For example, a cationic exchange resin-cross-linked with 4% divinyl benzene contracts during an infeed with molasses of 60.degree. Brix by approximately 30% compared to the resin in pure water.
This means that a resin column at a level of 9 meters shrinks to a level of almost 6 meters in pure water and the space above the resin bed becomes almost 3 meters greater.
It is not possible, therefore, to regulate the liquid level by means of level regulators, such for example, as a conductivity analyzer fixedly positioned at the top of the upper edge of the resin bed in pure water, since the incoming medium would be mixed with the solution above the resin bed in strongly contracted resin which would considerably impair the separating effect. One must therefore take particular care that the resin is constantly covered by liquid; however, the amount of liquid found above the resin surface prior to the addition of a liquid with another composition must be as small as possible.
These general rules are valid for all solid material beds which can be vertically altered. They are also especially valid for the attainment of sugar following the procedure of excluding ion in which sugar solutions for the separation of interferring components, pure water for elutriation and strong common salt solutions for regeneration are consecutively introduced to the column. If there is a high liquid level above the resin bed, various fractions which have been introduced, mix, whereby the separating effect of the column and thus the entire sugar output are impaired.
There is a known type of device which attempts to eliminate the detrimental influence of the change in volume of the resin bed. This device consists in the fact that the arrangement for the control of the operation of the valves which control the output of the crude washing waters treated by means of the ion exchanger and of the hot water to the column of the ion exclusion, has a volume control in each one of the inlet pipes and the volume control in its operation is controlled by a differential detector and an electric relay is operated, as long as a sequence switch in an electric switching network is present which is controlled by the relay while the operation of the volume control is regulated.
The aforesaid differential detector consists of a conductivity probe device attached in the middle of a float which gives a signal to an electric relay when the liquid level above the resin bed is less than 6.35 mm. It thus works in a noncontinuous fashion and indicates whether the medium surrounding it is electrically conductive or not. It is assumed here that the liquid is above the resin bed, as long as the analyzer indicates conductivity. This stipulation, however, is not true with foaming media, since the foam serves as a good conductor and thus the automatic control system is not interrupted at the right time, or not at all, which results in no further liquid being conducted to the column whereby the resin bed falls dry. If this occurs, the resin column must be rinsed back immediately, which results in operational interference.
This well-known controlling procedure is subject to further problems if solutions with extremely varying conductivity such as common salt or demineralized water are supplied to the column. If there is a small amount of sensitivity in the conductivity probes, the circuit is not closed during feeding with water; if the amount of sensitivity of the probe is too high, or if the liquid film clings to the float and the probe the relay will remain closed.
The present invention provides a method and apparatus for measuring and adjusting the liquid head over a bed of a solid material in a reliable fashion by means of a differential pressure detector, a regulating device and a controllable valve.